The present invention relates to an amplifier. Amplifiers that use transistors are known in the art. For example, Japanese Unexamined Patent Application Publication No. 2002-9564 discloses a variable gain device including a cascode amplifier circuit, an input matching circuit, and an output matching circuit. The cascode amplifier circuit includes a common-source first field effect transistor and a second field effect transistor. The source of the second field effect transistor is connected to the drain of the first field effect transistor, and the gate of the second field effect transistor is supplied with a gain control signal. The input matching circuit is connected between an input terminal for a high-frequency input signal and the gate of the first field effect transistor and can provide impedance that maximizes gain. The output matching circuit is connected between the drain of the second field effect transistor and an output terminal for a high-frequency output signal and sets the impedance seen from the drain of the second field effect transistor to a value that makes the phase deviation of a passing high-frequency signal substantially constant over a variable voltage range of the gain control signal. The disclosed variable gain device can keep the deviation of the passing phase substantially constant over a variable gain range. The disclosed variable gain device is low in power consumption, easy to reduce in size, and suitable for use in an IC.
The gain of an amplifier needs to be adjusted in accordance with the level of the signal to be input and it is desirable to linearly adjust the gain.
In the variable gain device disclosed in Japanese Unexamined Patent Application Publication No. 2002-9564, the gain is controlled in accordance with the voltage value of the gain control signal applied to the gate of the second field effect transistor. However, the range of voltage values of the gain control signal over which the gain can be controlled is limited to a linear area within which the current flowing through the second field effect transistor changes. For a voltage value that falls out of the linear area, the gain is not controllable. Therefore, in the variable gain device disclosed in Japanese Unexamined Patent Application Publication No. 2002-9564, if the voltage applied to the second field effect transistor falls below a certain value, the gain decreases excessively. As a result, it is difficult to finely adjust the attenuation of the gain.